Conventional geophysical surveying techniques employ several distinct technologies, which can include seismology, controlled-source electromagnetics, (CSEM), magnetotellurics, microseismology, gravity, magnetics, and controlled source electroseismology and seismoelectric surveying. Gravity and magnetics technologies survey large areas, such as whole geological basins. These technologies generally identify prospective regions with attractive geological features but do not generally identify the detailed location of hydrocarbon reservoirs or mineral resources. Microseismology generally relies on small, localized seismic events generated in the earth by naturally occurring earth movements or by well-drilling operations. Microseismology may locate the source of fracturing events such as encountered in fracturing reservoirs. Magnetotellurics uses the low-frequency portion of the earth's background electromagnetic fields to estimate the subsurface electrical conductivity but the method does not provide a detailed location or shape of target reservoir structures. Seismology produces information indicative of reservoir structures and CSEM provides electrical resistivity information that indirectly indicates the presence of hydrocarbons. Controlled source electroseismology and electroseismic surveying use high-power seismic or electromagnetic sources to create images of the subsurface that provide both structural and fluid property information. These methods are seldom used because they are significantly limited by the requirement for high-power sources.
In gravity surveying, sensitive gravity detectors are placed on the earth or above the earth. Reservoirs typically have smaller mass density than non-reservoir rock. The sensitive gravity meter detects a minimum in local gravitational acceleration over a reservoir. Gravity studies have several limitations. Local gravity values reflect an average of the mass densities from all materials in the neighborhood of the sensor. Whereas reservoirs of low density reduce the measured gravitational acceleration, the presence of high-density rock reduces the spatial resolution of the measurement and may obscure the presence of a low-density formation. The spatial resolution of gravity measurements is limited to length scales comparable to the depth and lateral extent of the reservoir. The amplitude of the identifying gravity signature depends on the volume of the reservoir. Little information is provided regarding the reservoir structure, pore-fluid properties, or the permeability.
In magnetic surveying, magnetic-field sensing devices measure the magnetic field of the earth, typically from aircraft. Hydrocarbon reservoirs and mineral deposits, such as iron ore, may alter the local earth's magnetic field. Measured data can be used to indicate the presence of reservoir structures. Magnetic surveying is limited because it measures neither properties related to the reservoir spatial extent and structure, nor the fluid identity and flow properties.
Seismic prospecting techniques generally involve the use of a seismic energy source and a set of receivers spread out along or near the earth's surface to detect seismic signals reflected from subsurface geological boundaries. These signals are recorded as a function of time, and subsequent processing of these signals is designed to reconstruct an appropriate image of the subsurface formation. In generic terms, this conventional process has seismic energy traveling down into the earth, reflecting from a particular geologic layer at a seismic impedance contrast, and returning to the receiver as a reflected seismic wave.
The seismic energy may be so-called shear waves (S-waves) or so-called compressional waves (P-waves). Shear waves and compressional waves differ with respect to their velocities, angles of reflection, vibrational directions, and to some extent the types of information that may be obtained from their respective types of seismic data. However, both types of waves suffer similar attenuation by earth formations; that is, the earth formations tend to attenuate the higher frequency components and allow the lower frequency components to pass through the earth relatively unattenuated. This means that, for deeper formations, the low frequency content of the reflected seismic energy contains the information about the underlying subsurface formations. However, because of the low frequency of the detected reflected seismic energy, the resolution of the reflected seismic energy may be insufficient to allow for detection of very thin geologic layers.
Further, if the seismic impedance contrast between adjacent but distinct geologic layers is small, little seismic energy is reflected and the distinctness of the geologic layers may not be discernible from the detected or recorded seismic data. Additionally, seismic studies might provide information about the structure of rock formations in the subsurface but generally are not able to distinguish between pore fluids, such as an aqueous fluid, oil, or gas.
Magnetotelluric surveying generally involves the use of the natural electromagnetic fields that originate in the earth's atmosphere. Naturally-occurring electromagnetic fields propagate into the subsurface where they encounter rock formations of differing electrical conductivity. When the electromagnetic fields contact a formation of low conductivity, such as is typical of hydrocarbon reservoirs, the electromagnetic field measured at the surface of the earth changes. Spatially-dependent electromagnetic fields measured on the earth's surface can be used to indicate the presence of low-conductivity formations that might contain hydrocarbons. Magnetotelluric surveying has several limitations. Only low-frequency, long-wavelength electromagnetic stimulation may reach prospective reservoirs because the high-frequency electromagnetic fields are rapidly attenuated by the conducting earth. Long-wavelength electromagnetic waves limit the spatial resolution of magnetotellurics making reservoir delineation difficult. Additionally, magnetotelluric surveying only provides information about formation electrical conductivity and does not yield data revealing information about porosity, permeability, or reservoir structure.
Seismic energy may be so-called micro-seismic energy. Seismic waves are generated in the earth by tectonic forces, by ocean tides and other natural phenomena. Seismic waves are also created when drilling or earth fracturing operations are conducted in hydrocarbon exploration, production, or in water well services. The events created by these natural and man-made events are called microseismic events. Generally, micro-seismic studies yield qualitative information about the location of subsurface structures or positional information about drilling operations. In these studies the location of the seismic source is imperfectly known so that only poor quality images of the subsurface are possible.
Controlled-source electromagnetic surveying involves the use of a source of electrical power and a set of electromagnetic receivers typically deployed on the seafloor in deep water. Although CSEM surveying may be done on land or in shallow water, recent work finds particularly useful applications in deep water. In CSEM surveying, the power source drives an electrical current into the earth that passes through the various subsurface rock formations. The electrical current follows a path of low electrical resistance through the most conducting rock masses. Hydrocarbon reservoirs contain insulating gas or oil fluids so the applied electrical current tends to flow around the resistive reservoir structures. The deflection of current around reservoirs is detected as a change in electromagnetic response on the detectors deployed on the seafloor. The measured signal properties can be used to reflect the presence of resistive reservoir structures.